The present disclosure relates generally to power supplies, and more particularly to regulation of voltage and/or current with primary-side sensing and feedback.
Power supplies, including battery chargers, may be used to deliver regulated voltage to electrical devices, including cell phones, tablets, power tools, and digital cameras, among numerous other examples. A power supply may be implemented using a commercially-available PWM (pulse width modulation) controller integrated circuit (IC). When employed in a battery charger, a PWM controller IC is configured to compensate for voltage drop on a charging cable connecting the battery charger to the battery, commonly referred to as cable drop compensation (CDC). In a conventional PWM controller, the cable voltage drop often necessitates the use of a dedicated cable compensation circuit and an extra, dedicated IC pin to attach additional electrical components for cable compensation to meet particular application specifications such as output current, output voltage, and cable resistance.
Generally, conventional cable drop compensation adjusts for the voltage drop across a cable caused by output current multiplied by the direct current resistance of the cable, commonly referred to as IR loss. As the output load current increases from 0% to 100% of the output current rating, IR losses increase from 0V to a product of the current at the maximum current rating and the resistance of the cable (IMAX×RCABLE).
FIG. 1 shows a simplified block diagram of prior art system 100 performing cable drop compensation. In FIG. 1, a power stage 102 delivers power to a load 104 via a cable, across which is generated a cable drop 106. The power stage 102 regulates power through the load 104 based on an adjusted feedback voltage Va. In the system 100, the adjusted feedback voltage Va is generated by adjusting a feedback voltage Vfb, representing a voltage across the load 104, by an estimated voltage drop Vreg across the cable. The estimated voltage drop Vreg is represented by the output current as a percentage of the maximum rated output current, multiplied by a CDC scaling factor CDC0. The CDC scaling factor CDC0 is a pre-set value that is specific to the output voltage setting, output load range, and estimated resistance 416 of the cable. Accordingly, the estimated voltage drop Vreg is linearly related to the output current, as shown in FIG. 2.
However, conventional cable drop compensation systems are based on the power supply providing a fixed regulated output voltage with a predefined output load range. These cable drop compensation systems do not address power supplies capable of delivering multiple regulated output voltage levels to a device. Furthermore, the minimum and maximum current outputs through the load may vary depending on the regulated output voltage setting. Conventional CDC systems adjust for cable IR losses of one output voltage and load range setting. But since the CDC scaling factor (CDC0) is a pre-set value and the actual IR losses across the cable are independent of the output voltage and load range, the cable drop compensation value will either over- or under-compensate for cable IR losses when the power supply is configured to a different output voltage and load ranges setting.
FIG. 3 illustrates that using the conventional pre-set CDC0 scaling factor will result in inaccurate CDC by either under-estimating or over-estimating the cable drop. FIG. 3 illustrates cable drop compensation for two output voltage settings V1 and V2, each having a corresponding current rating (e.g., the current range of the power supply operated at V2 is smaller than the current range at V1). When the same CDC0 scaling factor is used for both output voltage settings, the computed CDC value at a given output load I-LOAD has a CDC offset 302 that results in an inaccurate cable drop compensation value. For example, if the CDC scaling factor CDC0 is specified based on V1 while the power stage 102 is operated to output voltage at V2, the system undercompensates for the cable drop at I-LOAD by the offset 302.